Cefazolin, a widely used cephalosporin antibiotic in human therapy, was the target in this study. Manganese dioxide, which is abundant in soils and sediments, is one of the most important natural oxidants and catalysts. To better understand the fate of cephalosporin antibiotics in natural soils and sediments, the transformation of cefazolin with manganese dioxide (delta-MnO2) was investigated. Four major transformation products formed via the oxidation of thioether and olefin, hydrolysis and acid-catalyzed decarboxylation were identified, and plausible transformation pathways were proposed. The formation of transformation product species was found to be pH dependent due to the different oxidation abilities of delta-MnO2 under different pH conditions. The sulfoxide-type products formed in this study were previously reported to exhibit high genotoxicity. Therefore, the potential ecological risk of cefazolin transformation in natural sediments and soils is sufficient to merit concern. (C) 2017 Elsevier B.V. All rights reserved.